# Rational map of a curve to an elliptic curve

If I have a curve given by $$y^2 = (x^3-1)(x^3-a),$$ how do I find out if there is a rational variable transformation $y=y(s,t)$, $x=x(s,t)$ that maps this curve onto an elliptic curve of the form $$s^2+a_1st+a_3 s=t^3+a_2t^2+a_4t+a_6.$$ I think it's relevant that $x\mapsto a^{1/3}/x$ leaves the roots of the r.h.s. unchanged, but I don't know how I can use that. I learned that there should exist a quotient of the original curve by the involution $x\mapsto a^{1/3}/x$, but I am not clear how I would actually compute it.

Is there a way to explicitly determine the rational functions $y(s,t)$, $x(s,t)$?

## 1 Answer

I think that taking the quotient amounts to the following. I think about this at the level of function fields. So let $k$ be the field of constants. Then the function field of your curve is $F=k(x,y)$, which is a quadratic extension of the field of rational functions $K=k(x)$, where $y^2=(x^3-1)(x^3-a)$.

The symmetry that you talk about can be viewed as an automorphism $\phi$ of the field $F$. To avoid fractional exponents I denote $a=A^6$. Here I need to assume that $a$ has a sixth root in $k$. Hopefully this will not pose a problem. Now we can define the $k$-automorphism of $F$ by the rules $$\phi:x\mapsto\frac{A^2}x,\qquad y\mapsto \frac{yA^3}{x^3}.$$ We need to verify that this is an automorphism of $F$. This is equivalent to checking that $\phi(y)^2=(\phi(x)^3-1)(\phi(x)^3-A^6)$, and verifying that is straightforward.

You hinted at the possibility that we are moding out an involution. This is the case, because we easily see that $$\phi(\phi(x))=x,\qquad\text{and}\qquad\phi(\phi(y))=y.$$ Thus $\phi$ generates a cyclic group of order two. Basic Galois theory then tells us that $L=\operatorname{Inv}(\phi)$ is a subfield of $F$ such that $[F:L]=2$. Our next task is to identify the fixed field $L$. The first and most obvious observation is that $$u=x+\phi(x)=x+\frac{A^2}x\in L.$$ This is immediate from the involutory property of $\phi$. At this point we observe that $x$ is algebraic of degree two over the field $M=k(u)$. Thus we conclude that $M=L\cap K$.

Finding another "useful" element in $L$ was a bit tricky. It is easy enough to find elements from $L\setminus M$, but they were kludgy to work with. A choice that worked for me is $$v=\frac yx+\phi(\frac yx)=y\left(\frac1x+\frac A{x^2}\right).$$ Clearly $v^2$ will be an element of $M$ - we just need to calculate it \begin{aligned} v^2&=\frac{y^2}{x^2}\left(1+\frac Ax\right)^2=\frac{(x^3-1)(x^3-A^6)}{x^2}\left(1+\frac Ax\right)^2\\ &=\frac{A^8 + 2 A^7 x + A^6 x^2 - A^2 x^3 - A^8 x^3 - 2 A x^4 - 2 A^7 x^4 - x^5 - A^6 x^5 + A^2 x^6 + 2 A x^7 + x^8}{x^4}\\ &=u^4 + 2 A u^3 - 3 A^2 u^2 - (A^6 + 6 A^3 + 1) u-2(A^7+A).\\ \end{aligned}

It is easy to see that $L=M(v)=k(u,v)$. Evidently $k(u,v)\subseteq L$. Also clearly $y\in k(u,v,x)$, so $F=k(u,v,x)$ and we saw that $x$ is quadratic over $k(u,v)$ so $[F:k(u,v)]=2$. As $[F:L]=2$ the claim follows.

From the general theory of hyperelliptic curves we infer that the equation $$v^2=u^4 + 2 A u^3 - 3 A^2 u^2 - (A^6 + 6 A^3 + 1) u-2(A^7+A)\qquad(*)$$ defines a curve of genus $g=1$ whenever the roots of that quartic are distinct. Therefore $L$ is the function field of an elliptic curve whenever that holds.

The process of transforming the curve $(*)$ to a cubic needs to move one of the zeros of the r.h.s. to the infinity. Here we observe that $u=-2A$ is a zero of that quartic, so let's work with that. We can factor $$u^4 + 2 A u^3 - 3 A^2 u^2 - (A^6 + 6 A^3 + 1) u-2(A^7+A)=(u+2A)(u^3-3A^2u-A^6-1).$$ Here by Taylor expansion around $u=-2A$ $$u^3-3A^2u-A^6-1=(u+2A)^3-6A(u+2A)^2+9A^2(u+2A)-(A^3+1)^2.$$ This implies that dividing $(*)$ by $(u+2A)^4$ gives us, in the variables $$w=\frac{v}{(u+2A)^2},\qquad z=\frac1{u+2A},$$ the equation $$w^2=1-6Az+9A^2z^2-(A^3+1)^2z^3.\qquad(**)$$ Getting warmer! To get to the Weierstrass form we multiply $(**)$ by $(A^3+1)^4$ and introduce, finally, the variables $$s=(A^3+1)^2w,\qquad t=-(A^3+1)^2z,$$ whence the equation takes the form $$s^2=t^3+9A^2t^2+6A(A^3+1)^2z+(A^3+1)^4.$$

• Thank you very much. I was trying to find an integral of the form $\int dx/y$, and I read somewhere that I could relate it to $\int du/v$ by taking the quotient by the involution. It was a frustrating problem because I though it should be simple, but all the books I could find omitted the direct calculations entirely and I didn't know enough background to reconstruct them myself. Oct 12, 2014 at 19:26
• Looks good except for $-(A^3+1)z^3$: I get $-(A^3+1)^2z^3$ instead. Oct 12, 2014 at 19:34
• Better now? ${}$ Oct 12, 2014 at 19:38
• Yes. Thank you so much, this is a lot clearer to me now. Oct 12, 2014 at 19:44